An insect with only four legs

It’s an Aphaenogaster worker missing the metathorax and the propodeum. The mid-thorax is fused directly to the second abdominal segment, with the effect that the hind legs are just… gone. For comparison, have a look at a normal Aphaenogaster.

Myrmecologist Douglas Booher pulled her from a litter sample in Georgia. You’d think a major pair of walking legs would be indispensable, but this ant apparently was collected alive.

So workers don’t reproduce in ants, right? A trait like this wouldn’t get passed down (except by the same mechanism it was originally produced) which means, perhaps, that even if four legs were opitmal for workers, natural selection wouldn’t select it. Does that imply ants evolve more slowly than other animals? Actually, now I’m wondering how workers get desireable traits at all without being able to pass on genetic information.

Josh – The thing you forget is that the traits that determine worker morphology (or “fitness” in the sense of foranging success) are also present in their mother, the queen. As such queens with efficiently foranging workers will on average have a larger colony, thus producing more reproductive individuals (males, queens) and so the traits spreads.

Correct me if I’m wrong, but it seems to me that the rate of evolution is solely based on generation time. I guess I could look that up first, but I’ll just throw it out there anyway. You may not perceive as much change between generation of some populations, but with each generation they are evolving. I’ll have to think on this some more.

This is a surprisingly complicated question, and one that continues to occupy many professional biologists. The rate of evolution will depend on mutation rate, generation time, the strength & persistence of selection, amount of immigration, and population size. Here’s an online simulator, if you’d like to play with the parameters:

I didn’t think about nongenetic developmental problems. I confess I’m no expert in any particular science. Do worker ants have genetic variation, or are they straight clones? The explanations above make sense. Thanks!

Not entirely true. Some DNA mutates more rapidly than other DNA. Even specific parts of DNA either mutate more rapidly or slower than other parts. Also, the difference in selection pressure can change the rate. Nothing gets an advantage when everything gets to reproduce just as much as the next guy/girl.

Josh,

What you’re forgetting is that the queen produced the worker, so if it’s a genetic thing, then it could spread when she passes it on to the offspring that does reproduce. If it turns out to be a favorable trait, it could become widespread.

Oh, to clarify: the difference in selection pressure doesn’t change the mutation rate (at least, not that I know of). But if, for instance, the slowest 5% of ants die each generation, you’ll see a gradual increase in speed over time.

Right, but can the worker have a mutation that is not present in the queen? Is there any genetic variation in the workers? Seems to me that an if the queen has to pass along all the genetic information because the workers don’t reproduce, then any genetic variation in the workers (if there is any) would be glossed over. The gene transfer is only queen to worker, not the other way around. But I guess that one of the males is likely to have that genetic information too, and if it really grants a statistical advantage, then it’ll be selected for in his off spring, though how that genetic information gets passed on to subsequent generations seems to me to be little more than pure chance, since the gene “produce workers with three legs” doesn’t seem like it would help the male find a queen.

Hmm, actually, the more I think about this the more I realize my ignorance of how ant reproduction works. I’m sure it makes more sense when you get into the details. But it definitely seems to lack that straight logic as, say, a rabbit, where literally running faster might directly influence your ability to reproduce and spread the “running faster” genes.

The first picture shows an ant with eight legs! (I photographed it long ago, about in 1964, at Würzburg, Germany. The ant was alive, found in a natural nest).
Other specimens lack various parts of the body, together with he respective appendages.

Only the Harpagoxenus of Fig. 4 had been normal, but hat lost most of her appendages during colony foundation: As the (intermorphic!) queen of a slave maker species she had to fight against the defending workers of a host colony, Leptothorax acervorum. Despite these severe losses she eventually had successfully founded her colony which should have been about 3-4 years old when I detected it in a forest near Nuremberg, Germany.

Such developmental errors probably also occur in other insects. But an ant worker with 4 legs can survive because of “colonial buffering” – she emerges in a friendly environment, where she is protected and fed. She can survive at least until her first trip outside the nest.